Glass sheets carrying water based paint I

ABSTRACT

The invention is directed to a water-based paint composition comprising sodium silicate, water, water-soluble base, metal oxide pigment, and zinc oxide as an adhesion promoter. Preferably, it further comprises glass frit and surfactants. In other aspects, it is directed to the method of making the composition, and glass sheets having a cured paint field thereon, and the method of making such glass sheets.

This is a divisional of application Ser. No. 08/415,951 filed Mar. 24,1995 now U.S. Pat. No. 5,518,535, issued on May 21, 1996.

FIELD OF THE INVENTION

This invention is directed to water-based paint compositionsparticularly useful for providing a paint field on glass sheets. Thepaint composition comprises sodium silicate, water, water-soluble base,finely divided metal oxide pigment, and zinc oxide as an adhesionpromoter.

BACKGROUND OF THE INVENTION

Various paint compositions are known to skilled artisans for paintingautomotive and architectural glass. Such paints are used, for example,to form opaque borders around the peripheral marginal surfaces ofglazings used as windshields, sidelites, and backlites of motorvehicles.

Generally these compositions are ceramic compositions formed from amixture of metal oxides in an organic vehicle. The mixture of metaloxides is adjusted so as to achieve a specific color for the ultimatelyproduced fired ceramic paint. For example, the opaque peripheralmarginal paint bands fired onto automotive glazings are generally blackand may include oxides like chrome oxide, cobalt oxide and nickel oxide.The organic vehicles generally included in such compositions, e.g., pineoil, mineral oils, low molecular weight petroleum fractions and the likeare used to allow the ceramic paint to be applied to the glass surfaceby brushing, spreading, or screen printing. The metal oxides arenon-reactive with one another, as well as non-reactive with othermaterials like glass frits often contained in the ceramic paint. Theseglass frits are the materials which ultimately fuse the ceramic painttogether, and to the glass sheet, to insure that the ceramic paintremains affixed to the glass sheet after being cooled to roomtemperature. When these materials are applied to a glass sheet, they areheated at high temperatures generally above the softening point of theglass to cure the paint and make the painted glass sheets suitable forfurther handling during a latter high temperature shaping process.

In view of environmental and commercial considerations, it would bedesirable if water-based paints could be developed for use in place ofsuch organic vehicle paints. It would be further desirable to develop awater-based paint which could be cured at relatively low temperaturesrather than the high temperatures generally required for ceramic/organicvehicle paint curing. Subjecting the glass having the paint coating tosuch high temperatures during curing of the paint often impartundesirable optical distortions into the glass sheets. In order to besuitable as a replacement for the conventionally used ceramic paints,the water-based paint would need to provide a uniform coating, bedurable, and also adhere well to the glass.

It is an object of the present composition to provide a water-basedpaint composition which displays excellent adhesion to glass. It is afurther object of this invention to provide a water-based paint which iscurable at relatively low temperatures and which exhibits excellentdurability. Advantageously, the present invention water-based paintcomposition achieves these objectives and hence overcomes thedisadvantages of prior art ceramic/organic vehicle paints.

DISCLOSURE OF THE INVENTION

This invention is a water-based paint composition, which has excellentadhesion to glass. The composition comprises:

(i) water-soluble sodium silicate forming 20 to 40 weight percent of thecomposition,

(ii) water forming 5 to 25 weight percent of the composition;

(iii) water-soluble base in an amount sufficient to provide thecomposition with a pH of at least about 10.5;

(iv) finely divided metal oxide powder being selected from the groupconsisting of oxides of copper, iron, nickel, cobalt and mixturesthereof forming 25 to 40 weight percent of the composition and having aparticle size less than 7 microns; and

(v) zinc oxide forming 2 to 10 weight percent of the composition.

The composition preferably also comprises glass frit in low amounts. Theinvention is further directed to a process of making the disclosedwater-based paint which comprises combining and mixing the components.According to another embodiment of the present invention, it comprises aglass sheet having at least a portion coated with a cured paint madefrom the above disclosed composition. According to still anotherembodiment of the invention, it comprises a method for providing a curedcoating of the paint on a glass sheet.

Detailed Description of Preferred Embodiments

The invention composition is a water-based paint useful as a coating onglass, such as for the "black out" area around the periphery of awindshield. The composition comprises, as disclosed above, water-solublesodium silicate, water, water-soluble base, finely divided metal oxidepowder which gives the paint its black to grey color, and zinc oxide asan adhesion promoter. Each of these components, as well as optionalcomponents, will be discussed in detail below.

Water-soluble sodium silicates form between about 20 and 40 weightpercent of the composition of this invention, more preferably, betweenabout 30 and 36 weight percent of the composition, i.e., as used herein"weight percent of the composition" meaning a fraction of the totalweight of the composition. They may comprise a single sodium silicate ora mixture of silicates. Water-soluble sodium silicates useful in thisinvention composition can be represented by the formula SiO₂ :Na₂ O,wherein the mole ratio therein of the two oxides is from about 2:1 toabout 4:1. In addition to the water-soluble sodium silicate,water-soluble potassium silicates of similar formula may also beincluded in the composition. When they are included, they are generallypresent in minor amounts, preferably less than 10 percent by weight,generally between about 5 and 10 weight percent of the composition.

Yet another required component of the composition is water which isincluded in an amount between about 5 and 25 weight percent of the totalcomposition weight, preferably in an amount between about 10 and 20weight percent. The composition also includes a water-soluble base whichis employed to provide the paint composition with a pH of at least about10.5, preferably being above 12.5, more preferably about 13.5. The pH isrequired to be on the basic side as is indicated by the required pH.Providing the composition with this pH is necessary to impart desirableshelf life to the paint. Embodiments of invention paints with a pH aboveabout 13, for example, have been found to be shelf stable for at least 3months. Examples of water-soluble bases which may be employed includebut are not limited to sodium hydroxide and potassium hydroxide, withsodium hydroxide being preferred.

The amount of base employed is dependent, e.g., on the particular baseand concentration employed. For example, using a 1N sodium hydroxidebase, is generally included in about 2 to 10 weight percent of thecomposition, more preferably in about 3 to 8 weight percent, and mostoptimally, in about 3 to 6 weight percent. Optimal amounts and types ofbases which may be employed would be apparent to those skilled in theart in view of the present disclosure.

The composition further comprises finely divided metal oxide pigmentselected from copper oxide, iron oxide, nickel oxide, cobalt oxide andmixtures thereof, with the inclusion of copper oxide being preferred.This pigment gives the paint a black color and forms 25 to 40 weightpercent of the composition, preferably between about 25 to 35 weightpercent of the composition. The finely divided powder pigment has aparticle size (diameter) on average less than about 7 microns,preferably being between about 3 and 7 microns, most preferably beingabout 5 microns. These compositions would vary in color from black todark grey, depending on the mixture and percentages of the metal oxides.Copper oxide, which is preferred, provides a black paint composition. Itis undesirable to use chromium oxide in the composition since itinterferes with the adhesion of the paint to the glass.

Another essential component of the paint composition is a minor amountof adhesion promoter, i.e., zinc oxide in this invention paint. Thisadhesion promoter is included in the composition in an amount of betweenabout 2 and 10 weight percent, preferably between about 3 and 6 weightpercent, most preferably between about 4 and 6 weight percent, based onthe weight of the composition. Optimally, about 6 weight percent zincoxide is included in the composition. It is preferred that the zincoxide be provided in the paint composition in a particle size of betweenabout 2 and 3 microns on average, however particle size is not critical.While not wishing to be bound by theory, the inventor believes that thezinc oxide lowers the expansion coefficient of the silicate paintcomposition and makes it more nearly match that of the glasscomposition. Hence, during heating and cooling of the glass, less stressdevelops in the paint because the glass and paint are reasonably closelymatched in thermal expansion coefficients. Less stress in the paint isbelieved to result in good adherence of the paint to the glasssubstrate. Neither the understanding nor the accuracy of this theory ishowever necessary for the practice of this invention.

In addition to the above required components, the paint compositionoptionally, but preferably, comprises a small amount of low meltingpoint glass frit, i.e., generally comprising less than about 10 weightpercent, more preferably being between about 2 and 4 weight percent ofthe present invention blackish paint composition. Preferably, the lowmelting glass frit, when included, is present in the composition in anamount of about 3 weight percent, based on the total composition weight.By low melting glass frit is meant a glass material which melts belowabout 1300° F. Exemplary of preferred glass frit materials useful in thepresent composition, often referred to as enamel frit, are zincboro-silicate glass and bismuth boro-silicate glass, and mixturesthereof. Still other glass frits which may be employed in the presentinvention will be apparent to those skilled in the art in view of thepresent disclosure. I have found that including this frit material inthe glass composition further improves the adhesion of the paint to theglass.

Still another optional, but desirable, component included in the blackpaint composition of this invention is a surfactant. Surfactants arewell known materials and often added to paints to improve the wettingcharacteristics of the liquid paint to the substrate to which it isapplied. Exemplary of one such material is "FC-171" made by 3M Company.Still other surfactants are known to those skilled in the art.Desirably, it forms about 0.1 to 1.0 weight percent of the paintcomposition, more preferably about 0.25 to 0.5 weight percent.

To make the composition, the components are generally added together andthen balled milled until a substantially uniform mixture of componentsis obtained. This mixing is generally carried out at room temperature.Often, the base is added after the ball milling or in the final stagesof the milling. A water-based paint composition is commerciallyavailable which contains alkali silicates, water, base, and pigment likecopper oxide, i.e., CERAM-VUE™ (CO.1-112 Black, Industrial ControlDevelopment, inc, Vancouver, Wash.), which is a black water-basedsilicate paint of this type. If one adds the zinc oxide powder to thiscomposition at the levels indicated above, this composition after it iscured becomes more adherent to glass sheets on which it is applied.

After making the subject invention composition, it may be applied to asubstrate, particularly a glass substrate, by any technique. The glasssheet may be prepared from any type of glass generally known in the artof glass making. Typical glass sheets contemplated for use according tothe present invention are soda-lime-silica automotive and architecturalglazings, generally produced by the well-known float glass process.

In operation, the paint is applied as a uniform layer to a surface ofthe glass sheet in a predetermined pattern by a conventional paintapplication method, e.g., screen printing wherein the paint is spreadacross the screen by a squeegee to force the paint through the patternonto the glass sheet. It is well known in the painting arts to apply aband of paint to the surface of an automotive glazing by silk screening.In such a situation, it is particularly desirable to maintain a moistenvironment surrounding the paint during the screening process.Optimally the moist environment is maintained about 80+5% rh with thepresent invention preferred paint compositions. Maintaining this moistenvironment allows for prolonged use of the paint application screensystem by maintaining the moisture content of the paint at a desirableviscosity for application. This environment may be optimally provided,e.g, by the invention described in U.S. patent application Ser. No.08/295,574 filed Aug. 25, 1994 to Boaz and commonly assigned with thisinvention. It is entitled "Apparatus and Method for Applying a Coatingto Glass".

The predetermined pattern in which the paint may be applied to a glasssheet may comprise, for example, an opaque concealment band positionedon a peripheral marginal surface of an automotive glazing. Suchconcealment bands are well known in the art of automotive glazings asuseful for preventing the solar radiation induced degradation ofadhesives used to mount the glazings in a vehicle opening, and forconcealing attachment hardware and structural components which lie belowthe edges of the glazings. The band generally extends to the edges ofthe glazings, and has a width sufficient to conceal the underlyingadhesive and structural components, but which is sufficiently narrow toprovide maximum vision to occupants of the vehicle. Clearly, otherpredetermined patterns may be utilized where applying various paintfields to glass surfaces, depending upon the ultimate purpose for thecured paint field.

In the case of coatings applied to automotive glass as "black out", thecoating is preferably provided in a thickness of about 12-16 microns.The coating may be applied in any thickness, however, the optimalthickness being determined by the particular application desired.

The curable compositions of the invention, after being applied to asubstrate, are readily cured by baking at an elevated temperature for atime sufficient to drive off the water which cures the coating. Thisstep may be carried at any temperature, but desirably at a temperaturebelow the softening point of the glass. Since this water vaporizationand curing can be preferably carried out at moderate temperatures, e.g.,below about 400° C., even between about 100° C. and 200° C., the paintedglass is not subjected to softening and hence is prevented from beingdistorted during the moderate temperature.

This is in contrast to conventional organic vehicle paints which requirebeing heated to temperatures above the softening point of the glass tocure these paints. Paint compositions applied to glass sheets aregenerally first cured in order to make the painted glass available forhandling for further processing, i.e., bending of the painted glasssheet into a desired final shape. Heating glass to temperatures abovethe softening point of the glass to cure organic vehicle paints providesthe glass with an opportunity to pick up distortions. Subsequent bendingof the painted glass, in a second firing, again at these hightemperatures provides a second opportunity for distortions to beincorporated into the glass product. Hence, the present invention paint,because it can cure at relatively low temperatures below the softeningpoint of the glass, desirably minimizes the opportunity for distortionto be incorporated into the glass product.

This drying and curing of the water-based black paint composition may becarried out by any means. Two particularly preferred means comprisesubjecting the paint applied to the glass to Infra Red (IR) radiation ormicrowave radiation as in a microwave oven. The latter is mostparticularly preferred because it may be provided as a compact unit ofrelatively small size, it consumes less energy and generally requiresless maintenance.

In the case of a windshield type 12"×12" sample which had an embodimentof the invention paint composition applied to a portion of its surfaceas a "black out" band, it was found that the coating could be cured inabout one minute at 150° C. in an IR oven or about 1 minute in amicrowave oven (4 KW power). The particular embodiment of paintcomposition employed and the area coated suggest the optimal particularparameters useful to cure the invention paint.

As is evident, the present invention moderate temperature curablecompositions are of considerable advantage in that they provide forsignificant energy savings in comparison to compositions which requiresignificantly elevated temperatures for curing. In addition, asdiscussed above, when the substrate is glass, optical distortions can beincorporated into the glass sheet when it is exposed to significantlyelevated temperatures as required to cure conventional organic vehicleglass coatings. The present invention composition overcomes thisdisadvantage of prior art coatings which require elevated temperaturesfor curing the paint. While, as described above, the paint findsparticular usefulness on glass, it may also be used to paint othersubstrates, including, e.g., metal or plastic.

Generally, coated automotive glass will subsequently be subjected toshaping which involves subjecting the glass to high temperatures of theorder of 550° C. or more in a lehr. This will allow the coating tofurther cure, although such is not required to provide a durable andadherent coating on a substrate.

The following examples are presented by way of description of theinvention and to set forth the best mode contemplated by the inventor,but are not to be construed as limiting.

EXAMPLES

The following three examples are embodiments of paint compositions madeaccording to the present invention. All component amounts are in weightpercent of the total paint composition.

    ______________________________________                                        Paint #        1           2     3                                            ______________________________________                                        Sodium         32          32    25                                           Silicate                                                                      Potassium      4           4.5   13                                           Silicate                                                                      Water          20          19    16                                           Sodium         8           6     4                                            Hydroxide                                                                     (1 N)                                                                         Copper         32          32    34                                           Oxide                                                                         Zinc           2           4.0   4.0                                          Oxide                                                                         Frit           1.8         2.0   3.7                                          (zinc borosilicate)                                                           Surfactant     0.2         0.5   0.3                                          (FC-171                                                                       3M Company)                                                                   ______________________________________                                    

A paint composition (No. 4) was made according to an embodiment of thepresent invention by adding zinc oxide and surfactant (FC-171, 3MCompany) to a test sample water-based silicate paint (CB-555-195, A. O.Smith Company) in the following weight percent amounts of the totalcomposition: 93.54 silicate paint, 6.0 zinc oxide, and 0.51 surfactant.

The paint compositions above were applied by screen printing to glass(soda-lime-silica) sheets to a thickness of 12 microns and cured in anIR over for 3 minutes at 120° C.

The black cured paint displayed uniform coating and excellent adherenceto the glass as was shown by subjecting the painted glass to a hot waterbath for 5 days at 60° C. whereby the paint showed no delamination.Similar paint compositions without the zinc oxide adhesion promoter(hence not according to the present invention) are subject todelamination under similar test conditions.

The paints were also tested for adhesion to urethane type adhesives, bythe test described below, since these adhesive materials are often usedin contact with glass for mounting into a vehicle.

Urethane Adhesive/Paint/Glass Adhesion Test

The surface of a glass panel is painted and the paint fully cured bypassing it through an oven which heats the glass to over 550° C. Aportion (stripe) of the painted surface is prepared for urethaneadhesion by first applying a coating of Essex glass primer--Prep. No.43519. On this prepared surface, another layer of Essex glass primer No.43520A is applied. The primer is allowed to dry for about 10 minutes atroom temperature, and then a "bead" of Essex urethane polymer adhesiveis applied on the primer coatings. A second glass panel is prepared inidentical fashion except that the urethane adhesive is not applied. Thetwo glass panels are then pressed into contact so that the urethaneadhesive contacts the primer of the second panel, leaving about 0.25'between the painted surfaces of the panels. The pair is allowed to curefor 72 hours in air. To pass the adhesion test, the urethane and not theadhesive surface must break when the panels are twisted against eachother. The reliability of adhesion is tested after the adhered pair isimmersed in water at 150° C. for a chosen length of time.

The above paint compositions all showed excellent adhesion andsuccessfully passed the urethane adhesion test.

The paint compositions of the following examples, both according toembodiments of the present invention (#6 and #7) and comparative (#4 and#5) were tested for adhesion to urethane polymer adhesives as describedabove and also to polyvinyl butyral (used as an interlayer inwindshields) as follows:

Vinyl/Paint/Glass Adhesion "Pummel Test"

A 3" border of a paint is provided in a laminated pair of glass panelswith the paint being on a surface in contact with a polyvinyl butyralinterlayer. The laminate is cooled and stabilized at -10° C. for fourhours. At the end of the four hours, the laminate is quickly removed andthe painted portion placed over an anvil whereafter the portion issubjected to a five minute series of blows from a heavy hammer. At thistemperature, the vinyl is hard and has poor adhesion to the crushedglass particles. The adhesion of the glass particles to the vinyl israted by comparison to standards defined from #1 to #10, 3, #1indicating complete loss of adhesion between the vinyl/paint/glass and#10 indicating full adhesion of vinyl/paint/glass.

The following paints #4, #5, #6, and #7 contained relatively the samesodium silicate, potassium silicate, water, and pigment components.Additional components are listed for the particular examples (weightpercent). Paint #4: Basic composition: sodium silicate and potassiumsilicate, water, and pigment (copper oxide). Comparative example, notaccording to an embodiment of the present invention in that it does notcontain zinc oxide. Paint #5: Basic composition with 2% zincborosilicate frit. Comparative example, not according to an embodimentof the present invention in that it does not contain zinc oxide.

Paint #6: Basic composition with 2% zinc borosilicate frit and 5% ZnO,according to an embodiment of the present invention.

Paint #7. Basic composition with 5% zinc oxide, according to anembodiment of the present invention.

    ______________________________________                                        Results of Pummel Tested Vinyl/Paint/Glass                                    ______________________________________                                        #4    #1-#2 Pummel Test                                                                            #6 paint: #6-#8 Pummel Test                              paint:                                                                        #5    #4-#6 Pummel Test                                                                            #7 paint: #6-#8 Pummel Test                              paint                                                                         ______________________________________                                        Results of Urethane/Paint/Glass Adhesion Test, After:                                 4 Hours   24 Hours  48 Hours                                                                              120 Hpurs                                 ______________________________________                                        #4 paint                                                                              failed    --        --      --                                        #5 paint                                                                              passed    passed    passed  failed                                    #6 paint                                                                              passed    passed    passed  passed                                    #7 paint                                                                              passed    passed    passed  passed                                    ______________________________________                                    

It can be seen from the above test results, that the inclusion of zincoxide significantly improved the adhesion of paint, which is furtherimproved by the inclusion of the zinc borosilicate frit.

I claim:
 1. A glass sheet carrying on at least a portion thereof a curedcoating of a water-based paint composition, said compositioncomprising:(i) water-soluble sodium silicate forming 20 to 40 weightpercent of said composition, (ii) water forming 5 to 20 weight percentwater; (iii ) water-soluble base in an amount sufficient to provide saidcomposition with a pH of at least 10.5; (iv) finely divided metal oxidepowder being selected from the group consisting of oxides of copper,iron, nickel, cobalt and mixtures thereof forming 25 to 40 weightpercent of said composition and having a particle size between 3 and 7microns, and; (v) zinc oxide forming 2 to 10 weight percent of saidcomposition.
 2. The glass sheet according to claim 1 wherein saidcomposition further comprises less than about 10 percent by weight of aglass frit based on the total weight of said composition.
 3. The glasssheet according to claim 1 wherein said composition further comprisespotassium silicate.
 4. The glass sheet according to claim 1 wherein saidwater-soluble base is sodium hydroxide.
 5. The glass sheet according toclaim 1 wherein said composition further comprises 0.1 to 1.0 weightpercent surfactant.
 6. The glass sheet according to claim 1 being anautomotive or architectural glazing.